Internal Quality Control practice

The laboratory shall have quality control procedures for monitoring the validity of tests undertaken. The resulting data shall be recorded in such a way that trends are detectable and, where practicable, statistical techniques shall be applied to the reviewing of the results.

Quality control data shall be analysed and, where they are found to be outside pre-defined criteria, planned action shall be taken to correct the problem and to prevent incorrect results from being reported.

The control encompasses the analytical process starting with the sample entering the laboratory and ending with the analytical report. The most important tools in XRF quality control are the analysis of duplicate samples and the use of control charts. Blind duplicates are included in the analysis to monitor the precision, whereas reference materials are analyzed as control samples to monitor in control charts the long term stability in method performance. The results of each run (group of samples analyzed within a request for service) are then accepted or rejected if both IQC requirements are met.

When a quality control program is established, it is essential to have in mind the requirement on the analytical results and for what purposes the analytical results are produced (the concept of fit for purpose).

From the requirement on the analytical results the analyst sets up the control program:

• type of quality control sample,
  
  
• type of quality control chart,
  
  
• control limits (simple, warning and action limits),
  
  
• control frequency,
  
  
• amount of duplicates and control samples for each run.